diff --git a/IPython/frontend/qt/base_frontend_mixin.py b/IPython/frontend/qt/base_frontend_mixin.py
index 2a848cf..d9637a1 100644
--- a/IPython/frontend/qt/base_frontend_mixin.py
+++ b/IPython/frontend/qt/base_frontend_mixin.py
@@ -12,6 +12,7 @@ class BaseFrontendMixin(object):
     #---------------------------------------------------------------------------
     # 'BaseFrontendMixin' concrete interface
     #---------------------------------------------------------------------------
+    _kernel_client = None
 
     def _get_kernel_client(self):
         """Returns the current kernel client.